In pitches which are used as a raw material for producing carbon fibers having excellent strength and excellent modulus of elasticity, optical anisotropy is observed by a polarizing microscope. More specifically, such pitches are believed to contain a mesophase as described in U.S. Pat. No. 3,974,264. Further, it has recently been disclosed in Japanese Patent Application (OPI) No. 160427/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") that carbon fibers having a high modulus of elasticity can be produced with a pitch containing a neomesophase. By heating such pitches for a short time optical anisotropy is observed in them. Further, pitches used as a raw material for carbon fibers need not possess only optical anisotropy but must also be capable of being stably spun. However, it is not easy to produce pitches having both properties. In order to produce carbon fibers having excellent strength and excellent modulus of elasticity, it is not always possible to use any material as the raw material for making pitches. Materials having specified properties have been required.
It should be noted that in many published patents, for example, as described in U.S. Pat. Nos. 3,976,729 and 4,026,788, the raw material is not specified in the claims of patent specifications. Furthermore, such patents indicate that pitches used as a raw material for carbon fibers can be produced only by carrying out thermal modification of a wide variety of raw materials. However, according to the detailed descriptions and examples in such patents, the desired pitches can only be produced by using specified raw materials.
For example, U.S. Pat. No. 4,115,527 discloses that substances such as chrysene, etc., or tarry materials by-produced in high temperature cracking of petroleum crude oil are suitable for producing the pitch, i.e., a carbon fiber precursor, but conventional petroleum asphalts and coal tar pitches are not suitable. Further, U.S. Pat. No. 3,974,264 discloses that an aromatic base carbonaceous pitch having a carbon content of about 92 to about 96% by weight and a hydrogen content of about 4 to about 8% by weight is generally suitable for controlling a mesophase pitch. It has been described that elements excepting carbon and hydrogen, such as oxygen, sulfur and nitrogen, should not be present in an amount of more than about 4% by weight, because they are not suitable. Further, Example 1 of the same patent publication discloses that the precursor pitch has properties comprising a density of 1.23 g/cc, a softening point of 120.degree. C., a quinoline insoluble content of 0.83% by weight, a carbon content of 93.0%, a hydrogen content of 5.6%, a sulfur content of 1.1% and an ash content of 0.044%. Even if a density of 1.23 g/cc in these properties is maintained, it should be noted that it is difficult to obtain conventional petroleum heavy oil having such a high density. Examples as described in the other U.S. Pat. Nos. 3,976,729, 4,026,788 and 4,005,183 also disclose that the pitch is produced with a specified raw material.
The properties of heavy petroleum oils depend essentially upon the properties of crude oils from which they were produced and the process for producing the heavy oil. However, generally, it is rare that heavy oils having the suitable properties described in the above described Examples are produced, and, in many cases, they cannot be obtained. Accordingly, in order to produce carbon fibers industrially in a stabilized state, which have excellent strength and excellent modulus of elasticity with petroleum heavy oils, it is necessary to develop a process for producing a pitch wherein the finally resulting pitch has properties which are always within a specified range even if the properties of the raw material for the pitch vary.